[ben-schaaf]

There's really an endless list of these optimizations. A few I've used (though not necessarily in rust):

Atoms: Each string can be referenced with a single u32 or even u16, and they're inherently deduplicated.

Bump allocator: your strings are &str, allocation is super fast with limited fragmentation.

Single pointer strings (this has a name, I can't think of it right now): you store the length inside the allocation instead of in each reference, so your strings are a single pointer.

[tialaramex]

ColdString is both your "Single pointer string" and a Small String Optimisation on top.

First, on the heap we have a self-indicating length prefix, basically we use the bottom 7 bits of each byte to indicate 7 bits of length and the top bit indicates there are more bits in the next byte. So "ben-schaaf" would be 0x0A then the ASCII for "ben-schaaf"

But, we avoid even having a heap allocation if we have 8 or fewer UTF-8 bytes to encode the text, that's our Small String Optimisation.

To pull this off we specify that our heap allocations will have 4 byte alignment even though they don't need it. This shouldn't be a problem, in fact many allocators never actually deliver smaller alignments anyway.

This means our pointer now has two spare bits, the least significant bits are now always zero for a valid heap pointer. We rotate these bits to the top of the first byte (this varies depending on whether the target is big-endian or little-endian) and we mask them so that for these valid pointers they are 0b10xxxxxx

So, now we can look at the "single pointer" and figure out

If it begins 0b10xxxxxx it really will be a valid pointer, rotate it, mask out that flag bit and dereference the pointer to find the length-prefixed text.

If it begins 0b11111AAA there's a short string here but it didn't need all 8 bytes, the next AAA bytes of the "pointer" are just UTF-8 and conveniently AAA is enough binary for 0 through 7 to be signalled, the exact length we have

If it has any other value the entire 8 bytes of "pointer" is a UTF-8 encoded string

[VorpalWay]

Storing the length in the allocation has potential performance tradeoffs too! The most obvious is that taking a substring/string view will need a copy (or use a different type that store the length outside the allocation).

But it also means the CPU has to follow the pointer (and potentially get a cache miss or pipeline stall) to find the length. Having a fat pointer of ptr+length makes a lot of sense for string views, and for owned string buffers with capacity it can mean avoiding a cache miss when appending to the buffer.

It's complicated in other words.

[Rendello]

> Bump allocator

You can build an ad-hoc bump allocator by using a String and indexing into it. You can't use &str references though, as a growing String may reallocate elsewhere and invalidate your references (Rust won't even let you try this), so you have to use your own indices. This is the same thing that bump allocator libraries usually do, too. It can be tricky but have great performance gains.

I recently 100x-d the speed of an XML/HTML builder I use internally by rewriting it to only have one thing on the heap, a single String. Every push happens right at the call site linearly, and by passing data through closures the formatting (indentation, etc.) is controllable. My first iteration was written in the least efficient way possible and had thousands of tiny allocations in nested heap objects, it was painfully slow.

[MrBuddyCasino]

Atoms: is this similar to interned strings?

[locknitpicker]

> Atoms: is this similar to interned strings?

Yes. It is exactly how they are described.

https://docs.rs/string_cache/latest/string_cache/struct.Atom...

> Represents a string that has been interned.

[ben-schaaf]

The names of these things are hazy and inconsistent. In Java and C# an interned string is the same type as other strings. Others describe atoms as interned strings, some call them symbols. At my work we call the u16/u32 atoms and interned strings are the single pointer strings described above.

[locknitpicker]

> There's really an endless list of these optimizations.

These aren't really optimizations. They are specialized implementations that introduce design and architectural tradeoffs.

For example, Rust's Atom represents a string that has been interned, and it's actually an implementation of a design pattern popular in the likes of Erlang/Elixir. This is essentially a specialized implementations of the old Flyweight design pattern, where managing N independent instances of an expensive read-only object is replaced with a singleton instance that's referenced through a key handle.

I would hardly call this an optimization. It actually represents a significant change to a system's architecture. You have to introduce a set of significant architectural constraints into your system to leverage a specific tradeoff. This isn't just a tweak that makes everything run magically leaner and faster.

[dryarzeg]

> everything run magically leaner and faster

In my opinion, there's no magic in the software engineering. Everything (or almost everything) is a system that can be described, explained, modified and so on. Applications, libraries, operating systems, kernels, CPUs/RAM/GPU/NPU/xPU/whatever silicon there is, ALUs/etc, transistors, electricity, physics... That's nowhere near "magic". There's always some trade-offs, it's just that you may not be aware of them initially.

[ben-schaaf]

You might want to refresh your understanding of the word optimisation. Changing a system to be more effective/efficient is optimisation, how big that change is makes no difference.